Numerous methods exist for measuring the thickness of a moving web or sheet, such as paper. Two of the most common techniques include a direct thickness measurement using contacting glides or shoes, which skim along the two surfaces of the web, and a non-contacting inferential method in which radiation absorption by the web is used to determine the weight per unit area of the web and the thickness is thereafter inferred, provided the density of the material is known with sufficient precision. Many variations and improvements to these methods exist, but each of the techniques has underlying drawbacks.
The contacting method is subject to three fundamental types of problems. First, the method can be limited by the strength of the material being measured. With fragile sheets such as tissue, for example, there is a tendency for the contacting shoes to snag deviations in the sheet surface, causing flaws in the sheet or even causing the sheet to tear. Second, the sheet itself can damage a contacting caliper sensor due either to abrasive wear on the contacting elements or to physical damage arising during sheet breaks. For caliper sensors that traverse the sheet, damage can also be caused when the sensor crosses the sheet edge. Third, the accuracy of contacting sensors can be adversely affected by the buildup of contaminants on the contacting elements, as may occur with coated or filled sheets or sheets containing recycled materials.
The non-contacting inferential thickness measurement methods avoid many of the problems of the contacting methods, but are subject to a new set of problems. For example, radioactive sources, which are common for thickness measurements when the density of the product is known, are not permitted in some web markets. Also the radioactive measurement is inferential, which means that if the density of the web is not as predicted, there may be significant errors in the calculated thickness value.
Several patents have suggested that use of lasers to measure the thickness of a moving web may be a promising option compared to the other methods available. One such system in U.S. Pat. No. 5,210,593 to Kramer and another such system described in U.S. Pat. No. 4,276,480 to Watson. In both these systems, the laser caliper apparatus comprises a laser source on either side of the web, whose light is directed onto the web surface and subsequently reflected to a receiver. The characteristics of the received laser signal are thereafter used to determine the distance from each receiver to the web surface. These distances are added together, and the result is subtracted from a known value for the distance between the two laser receivers. The result represents the web's thickness.
The above non-contacting approaches to thickness measurement have the desirable feature that they eliminate many of the disadvantages of the contacting method and the non-contacting inferential methods. However, there are difficulties with previous non-contacting techniques that can limit their use to relatively low-accuracy situations.
One of the problems is that the web may not always be perpendicular to the incident light, since the web has a tendency to bounce or develop intermittent wave-like motion. If the web is non-perpendicular to the incident light and the light beams from two opposing light sources are not directed to exactly the same spot on the sheet, substantial error in measurement can occur. This is caused by a number of factors. First, actual web thickness variations from the first laser's measurement spot to the second laser's measurement spot can cause an incorrect thickness measurement. Second, if the web is not perpendicular to the incident light, the measurement technique will cause an error in the thickness value proportional to the web's angle and to the displacement on the sheet surface between the two measurement spots. Bouncing or oscillation of the web can further exacerbate this error.
Another issue with the projection of laser light on to a paper or thin film is that the surface is usually not well defined and the paper or film is semi-transparent. Current laser triangulation sensors which use position sensitive detector such as Lateral Effect Detectors (LEP), Quad-cell or Bi-cell detectors contain insufficient information that is needed to extract the ‘true’ surface location of the surface.
Various minute changes in system geometry caused, in part, by thermal effects on physical dimensions of the measurement apparatus or by device calibration, can also degrade the measurement accuracy. These effects can be difficult to quantify directly, for example, by measuring temperatures at various points in the apparatus and applying appropriate correctors. These adverse effects become substantially more significant as the accuracy level of the measurement device approaches that required for the measurement and control of products such as newsprint or other thin products.